The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems are used to provide fresh water from brackish or sea water. A membrane is used that restricts the flow of dissolved solids therethrough.
A reverse osmosis system involves pressurizing a solution with an applied pressure greater than an osmotic pressure created by the dissolve salts within the solution. The osmotic pressure is generally proportional to the concentration level of the salt. The approximate osmotic pressure in pounds-per-square-inch is the ratio of the salt mass to water mass times 14,000. A one-percent solution of salt would have an osmotic pressure of about 140 psi. Ocean water typically has a 3.5 percent concentration and an osmotic pressure of 490 psi.
Water extracted from a reverse osmosis system is called permeate. As a given body of saline solution is processed by the reverse osmosis membrane, the concentration of the solution is increased. At some point, it is no longer practical to recover permeate from the solution. The rejected material is called brine or the reject. Typically, about 50% of recovery of permeate from the original volume of sea water solution reaches a practical recovery limit.
Referring now to FIG. 1, a reverse osmosis system 10 is illustrated having a membrane array 12 that generates a permeate stream 14 and a brine stream 16 from a feed stream 18. The feed stream 18 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to the required pressure flow which enters the membrane array 12.
The permeate stream 14 is purified fluid flow at a low pressure. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. The membrane array 12 requires an exact flow rate for optimal operation. A brine throttle valve 30 may be used to regulate the flow through the membrane array 12. Changes take place due to water temperature, salinity, as well as membrane characteristics, such as fouling. The membrane array 12 may also be operated at off-design conditions on an emergency basis. The feed pumping system is required to meet variable flow and pressure requirements.
In general, a higher feed pressure increases permeate production and, conversely, a reduced feed pressure reduces permeate production. The membrane array 12 is required to maintain a specific recovery which is the ratio of the permeate flow to feed flow. The feed flow or brine flow likewise requires regulation.
A pretreatment system 21 may also be provided to pre-treat the fluid into the membrane array 12. The pretreatment system 21 may be used to remove solid materials such as sand, grit and suspended materials. Each of the embodiments below including those in the detailed disclosure may include a pretreatment system 21.
Referring now to FIG. 2, a system similar to that in FIG. 1 is illustrated with the addition of a feed throttle valve 30. Medium and large reverse osmosis plants typically include centrifugal-type pumps 20. The pumps have a relatively low cost and good efficiency, but they may generate a fixed pressure differential at a given flow rate and speed of rotation. One way prior systems were designed was to size the feed pump 20 to generate the highest possible membrane pressure and then use the throttle valve 30 to reduce the excess pressure to meet the membrane pressure requirement. Such a system has a low capital cost advantage but sacrifices energy efficiency since the feed pump generates more pressure and uses more power than is required for a typical operation.
Referring now to FIG. 3, another system for solving the pressure/flow characteristics is to add a variable frequency drive 36 to operate the motor 22 which, in turn, controls the operation of the feed pump 20. Thus, the feed pump 20 is operated at variable speed to match the membrane pressure requirement. The variable frequency drives 36 are expensive with large capacities and consume about three percent of the power that would otherwise have gone to the pump motor.
Referring now to FIG. 4, a system similar to that illustrated in FIG. 1 is illustrated using the same reference numerals. In this embodiment, a hydraulic pressure booster 40 having a pump portion 42 and a turbine portion 44 is used to recover energy from the brine stream 16. The pump portion 42 and the turbine portion 44 are coupled together with a common shaft 46. High pressure from the brine stream passes through the turbine portion 44 which causes the shaft 46 to rotate and drive the pump portion 42. The pump portion 42 raises the feed pressure in the feed stream 18. This increases the energy efficiency of the system. The booster 40 generates a portion of the feed pressure requirement for the membrane array 12 and, thus, the feed pump 20 and motor 22 may be reduced in size since a reduced amount of pressure is required by them.
Referring now to FIG. 5, a membrane element 60 that is suitable for positioning within a membrane array 12 of one of the previous Figs. is illustrated. The element 60 includes leaves of membrane material wrapped into a spiral configuration and placed in a thin tube 62 of material such as fiberglass. Each membrane leaf includes two membrane sheets glued on three sides with the fourth side attached to a central permeate pipe 64. Spacer grids (not shown) keep the membrane sheet from collapsing under the applied pressure. Feed solution enters one end of the membrane array 60 in the direction indicated by arrows 66. The solution or feed flows axially along the membrane element 60 and between the leaves 68 and exits through the high pressure brine outlet as indicated by arrows 70. Permeate is collected from the leaves 68 through permeate pipe 64. The pressure of the permeate through the tube 64 is essentially zero since the applied pressure is used to overcome the osmotic pressure and frictional losses of the flow of feed material through the membrane.
Referring now to FIG. 6, a pressure vessel 78 that includes a plurality of membrane elements referred to collectively with reference numeral 60 is illustrated. In this example, three membrane elements are disposed within the pressure vessel 78. Each is denoted by a numerical and alphabetical identifier. In this example, three membrane elements 60a, 60b and 60c are provided in the pressure vessel 78. The pressure vessel 78 includes a first end cap 80 at the input end and a second end cap 82 at the outlet end. Feed is introduced into the pressure vessel in the direction of the arrows 84.
In this example, the three membrane elements 60a-60c are placed in series. Each subsequent element extracts a smaller amount of permeate than the preceding element due to an increasing osmotic pressure and decreasing applied pressure caused by frictional losses within the membrane elements. As a consequence, the final element 60c may produce very little permeate. The permeate pipe 64 collects permeate from each of the membrane elements 60a-60c. 
A typical reverse osmosis system operates at a constant pressure that is developed at the feed pump 20. The result is that an excess of applied pressure at the first membrane array may result in an undesirably high rate of permeate extraction which may allow the membranes to be damaged. The final membrane element 60c may have an undesirably low rate of extraction which may result in permeate with an excessive amount of salt contamination.
Referring now to FIG. 7, a manually operated reverse osmosis system 100 is illustrated. A reservoir 102 may be filled with seawater or other brine solution. A manually operated pump 104 having a lever 106 draws feed from the reservoir 102. The pump 104 raises the pressure of the feed and provides the pressurized feed to a pressure vessel 108 having a membrane 110 therein. Permeate produced through the membrane exits the pressure vessel 108 through a permeate pipe 112. Brine, under high pressure, leads the pressure vessel 108 through a brine pipe 114. A control valve 116 is used to reduce the pressure of the high-pressure brine stream in the brine pipe 114. Once the pressure is reduced in the brine stream through the pressure-reducing valve 116, the brine stream enters a drain 118.
A control valve 122 may be used to control adjust the feed pressure and flow required for proper operation of the membrane 110.
Referring now to FIG. 8, another embodiment of a manually operated reverse osmosis system 100′ is illustrated. Many of the components are similar to those set forth in FIG. 7 and thus are provided the same reference numerals. In this embodiment, an energy recovery device 140 is used in the brine stream. The energy recovery device 140 receives the brine from the high-pressure brine pipe 114. The energy recovery device 140 has a recirculation device such as a piston assembly 138 that includes a brine piston 142 that is connected to a feed piston 144 with a connecting rod 146. The brine piston 142 may have a larger diameter than the feed piston 144 to accommodate pressure losses that occur in the membrane 110 and the interconnecting piping.
The energy recovery device receives feed through a feed pipe 150 which is in fluid communication with the fluid reservoir 102. Fluid enters the energy recovery device 140 through a one-way valve 152. Under pressure from within the energy recovery device 140, the valve 152 closes.
The energy recovery device 140 has an outlet feed pipe 154 that is in fluid communication with the energy recovery device 140 through a valve 156. The valve 156 may also be a one-way valve. The valve 156 operates in the opposite direction than that of valve 152. For example, when the pressure in the feed line 150 is higher than the pressure within a chamber 160, adjacent to the piston 144, feed fluid is input into the chamber 160. When pressure is high within the chamber 160, the valve 152 closes and valve 156 opens and provides a higher-pressure feed into the pressure vessel 108. Valve-timing equipment 164 admits high-pressure brine to the energy recovery device 140 resulting in movement of the brine piston 142 which causes the feed piston 144 to increase the pressure of the feed into the high-pressure feed manifold 166. A shaft seal 168 seals the connecting rod 146 to prevent losses between the high-pressure side and the low-pressure side of the energy recovery device 140. When the pumping stroke is complete, the piston assembly is moved in the opposite direction so that new feed is admitted into the chamber 140 through the valve 142. This causes the brine to be ejected through the drain 118. A motor or other actuator may be used to move the piston in the reverse direction. The actuator has not been shown to simplify the drawing. The energy recovery device 140 only pressurized a feed flow equal to the permeate flow. This eliminates work otherwise needed to pressurize the flow that will be rejected as high-pressure brine.
The embodiments of FIGS. 7 and 8 may also be provided with a multiple membrane element pressure vessel as described in FIG. 4.
One aspect of prior reverse osmosis systems is that the process operates at a constant pressure developed by the feed pump. Examples of this are illustrated in FIGS. 1-4. The result of a constant pressure feed pump is that excess pressure is applied at the beginning of the membrane array where the osmotic pressure is relatively low. This may result in an undesirably high rate of permeate extraction which can damage the membranes. On the other hand, the final membrane element where the osmotic pressure is high may have an undesirably low rate of extraction which may result in a permeate with an excessive amount of sale contamination.